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Assuming we're all on the right path we have to remember that whatever the total expansion is effectively only have will rise up into the header tank and half the vent as we know the level in both is equal. Like John said he would have to raise his tank roughly another 5 metres to avoid this problem. The average system has a f and e pipe roughly 2 metres which when 15mm holds 0.29 litres, so with a system volume of 100 litres that's roughly 2 litres of added volume. Even with a 22mm f and e (which is rare in the average domestic systems) that only accommodates .64 litres, so there will always be some system water rising into the tank, especially when you consider that the hotter less dense water will rise to the top and the cooler more dense will fall. We're all bang on with the maths, I just hope we're not missing something.